You have to watch alpine units...
There are two interconnects M-Bus and the newer Ai-net
the CHM chamger is M-Bus (indicated by the M in the name) the other one will be Ai net for the same reason (A at the end of the name).
For example I've put an CDA7893R head unit in my 156 so that would require an AiNet changer.
I don't know what the model of HU that is fitted as standard by alfa but if it takes an M-Bus changer it ends in an M so wont take a changer that ends in an A....
Of course alpine do an M-Bus to Ainet interface unit so you can use M-Bus changers with AI-net head units, but not the other way around...
Hope this helps,
Alfa V6 - the ultimate aural pleasure
Simes - the Alfaowner formerly known as Alfa156man